Makes 36-ish bars
1 19.8 ounce package of brownie mix (or enough for a 13x9" brownie pan, we used Duncan Hines)
Preheat oven to 350 degrees Fahrenheit. Lightly butter or oil a 13x9x2" brownie baking pan.
In a large bowl, combine the eggs, oil and vanilla extract and whisk well. Add the brownie mix and stir until combined. Add the chopped peanuts. Remove half the mixture to a smaller bowl and set aside.
Spread the remaining half the brownie-peanut mixture into the pan first using a spoon and then using your hands to flatten evenly.
In a medium bowl, using a rubber spatula or spoon, mix together the sweetened condensed milk and the peanut butter until smooth. Gently spread the mixture over the first layer of brownie until evenly covering the brownie mix.
Break the remaining brownie-peanut mix into pieces and flatten with your hands and slowly and gently spread over the peanut butter mixture. It will be a bit messy. Use your hands to coax the final brownie layer into the corners and covering the peanut butter.
Finally, bake the brownies for 25-30 minutes until the top is set and the edges are lightly browning. Cool on a wire rack before cutting and serving.